Interest on a joint account in the UK is attributed to both account holders with an equal 50/50 split, regardless of who has deposited the account balance.
If next year you become a higher rate tax payer but your girlfriend remains on the basic rate, you would have to pay the full 40% rate of tax on half of the interest.
E.g. You have a balance of 100 pounds on which you receive 5 pounds in interest. 2.50 is attributable to you and 2.50 to your girlfriend. The bank will deduct 1 pound in interest automatically (20%), 50p tax each. Your girlfriend has already paid all tax due. Your 2.50 is taxable at 40% so you owe the taxman another 50p which you will have to declare in your tax return.
Another way to think of it is that for a joint account where only one account holder is a higher rate payer you will owe 30% tax instead of the full 40%.
If you're happy for the account to be held only in your girlfriend's name then she will be taxed at 20% on the whole balance.
c.f. worked example from HMRC